Assertion :Muscles fibres of SA node possess the lowest rhythmicity among all cardiac muscles. Reason: Due to this fact, it can initiate excitory waves at the highest rate.

Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.

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Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.

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Assertion is correct but Reason is incorrect.

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Both Assertion and Reason are incorrect.

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Solution

The correct option is D Both Assertion and Reason are incorrect.
SA (sinoatrial) node and AV (atrioventricular) nodes are contraction nodes in the mammalian heart. SA node muscle fibers have the highest rhythmicity among all cardiac muscles fibers because of which they can initiate excitatory wave at the highest rate. Therefore, cardiac impulses normally originate from SA node. Thus, the correct answer is option D.

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